Masked ion implant with fast-slow scan

ABSTRACT

An improved method of producing solar cells utilizes a mask which is fixed relative to an ion beam in an ion implanter. The ion beam is directed through a plurality of apertures in the mask toward a substrate. The substrate is moved at different speeds such that the substrate is exposed to an ion dose rate when the substrate is moved at a first scan rate and to a second ion dose rate when the substrate is moved at a second scan rate. By modifying the scan rate, various dose rates may be implanted on the substrate at corresponding substrate locations. This allows ion implantation to be used to provide precise doping profiles advantageous for manufacturing solar cells.

RELATED APPLICATIONS

This patent application claims priority to U.S. Provisional PatentApplication Ser. No. 61/232,821 filed Aug. 11, 2009 which is herebyincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention relate to the field of device fabrication.More particularly, the present disclosure relates to a scanning methodfor ion implantation utilizing a shadow mask.

2. Discussion of Related Art

Ion implantation is a standard technique for introducingconductivity-altering impurities into substrates. A precise dopingprofile in a substrate and associated thin film structure is criticalfor proper device performance. Generally, a desired impurity material isionized in an ion source, the ions are accelerated to form an ion beamof prescribed energy, and the ion beam is directed at the surface of thesubstrate. The energetic ions in the beam penetrate into the bulk of thesubstrate material and are embedded into the crystalline lattice of thesubstrate material to form a region of desired conductivity.

Such an ion implanter may be used to form solar cells. Solar cells aretypically manufactured using the same processes used for othersemiconductor devices, often using silicon as the substrate material. Asemiconductor solar cell has an in-built electric field that separatesthe charge carriers generated through the absorption of photons in thesemiconductor material. This electric-field is typically created throughthe formation of a p-n junction (diode) which is created by differentialdoping of the semiconductor material. Doping a part of the semiconductorsubstrate (e.g. surface region) with impurities of opposite polarityforms a p-n junction that may be used as a photovoltaic deviceconverting light into electricity. These solar cells providepollution-free, equal-access energy using a recurring natural resource.Due to environmental concerns and rising energy costs, solar cells arebecoming more globally important. Reducing cost to manufacture orincreasing production capability of these high-performance solar cellsor other efficiency improvement to high-performance solar cells wouldhave a positive impact on the implementation of solar cells worldwide.This will enable the wider availability of this clean energy technology.

Solar cells may require doping to improve efficiency. This may be seenin FIG. 1 which is a cross-sectional view of a selective emitter solarcell. It may increase efficiency of a solar cell to dope the emitter 200and provide additional dopant to the regions 201 under the contacts 202.More heavily doping the regions 201 improves conductivity and havingless doping between the contacts 202 improves charge collection. Thecontacts 202 may only be spaced approximately 2-3 mm apart. The regions201 may only be approximately 50-300 μm across.

FIG. 2 is a cross-sectional view of an interdigitated back contact (IBC)solar cell. In the IBC solar cell, the junction is on the back of thesolar cell. The doping pattern is alternating p-type and n-type dopantregions in this particular embodiment. The p+ emitter 203 and the n+back surface field 204 may be doped. This doping may enable the junctionin the IBC solar cell to function or have increased efficiency.

In the past, solar cells have been doped using a dopant-containing glassor a paste that is heated to diffuse dopants into the solar cell. Thisdoes not allow precise doping of the various regions of the solar celland, if voids, air bubbles, or contaminants are present, non-uniformdoping may occur. Solar cells could benefit from ion implantationbecause ion implantation allows precise doping of the solar cell. Ionimplantation of solar cells, however, may require a certain pattern ofdopants or that only certain regions of the solar cell substrate areimplanted with ions. Previously, implantation of only certain regions ofa substrate has been accomplished using photoresist and ionimplantation. However, use of photoresist, adds an extra cost to solarcell production because extra process steps are involved. Other hardmasks on the solar cell surface are expensive and likewise require extraprocess steps. There are advantages in implanting small regions of solarcells and having a lower sheet resistance between implanted regions toimprove series resistance. Both may be accomplished through the use ofion implantation. Accordingly, there is a need in the art for animproved method of implanting through a shadow mask and, moreparticularly, a scanning method for ion implantation that uses a shadowmask with solar cell fabrication.

SUMMARY OF THE INVENTION

Exemplary embodiments of the present invention are directed to anapparatus and method for implanting ions into a substrate in an ionimplanter. In an exemplary method, an ion beam is directed through anaperture of a shadow mask toward a substrate support configured tosupport a target substrate. A first portion of the substrate alignedwith the aperture of the mask is exposed to the ion beam. The substratesupport is moved with respect to the ion beam at a first scan rate whenthe first portion of the substrate is exposed to the ion beam. A secondportion of the substrate aligned with the aperture of the mask isexposed to the ion beam. The substrate support is moved with respect tothe ion beam at a second scan rate when the second portion of thesubstrate is exposed to the ion beam, wherein the first scan rate andthe second scan rate are different.

In an exemplary embodiment, an ion implanter includes an ion source, abeam line assembly, a shadow mask and a scanning assembly. The beam lineassembly is configured to extract ions from the ion source to form anion beam and direct the ion beam toward a substrate disposed on asubstrate support. The mask is disposed in front of the substrate andhas a plurality of apertures to allow respective portions of the ionbeam through the mask toward the substrate. The scanning assembly isconfigured to move the substrate with respect to the ion beam at a firstscan rate when first portions of the substrate are aligned with theplurality of apertures and at a second scan rate when second portions ofthe substrate are aligned with the plurality of apertures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a selective emitter solar cell;

FIG. 2 is a cross-sectional view of an interdigitated back contact solarcell;

FIG. 3A is a block diagram of a representative ion implanter inaccordance with an embodiment of the present disclosure;

FIG. 3B is a cross-sectional view of ion implantation through a shadowmask;

FIG. 4 is a front perspective view of a shadow mask;

FIG. 5 is example of a velocity profile for a substrate in accordancewith an embodiment of the present disclosure;

FIGS. 6A-6E represent the implant method corresponding to the velocityprofile of FIG. 5 in accordance with an embodiment of the presentdisclosure; and

FIG. 7 is a front perspective view of a substrate that results from theimplant method illustrated in FIGS. 6A-6E in accordance with anembodiment of the present disclosure.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention, however, may be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thedrawings, like numbers refer to like elements throughout.

FIG. 3A is a block diagram of an ion implanter 115 including an ionsource chamber 120. A power supply 121 supplies the required energy tosource chamber 120 which is configured to generate ions of a particularspecies. The generated ions are extracted from the source through aseries of electrodes 114 and formed into a beam 101 which passes througha mass analyzer magnet 116. The mass analyzer is configured with aparticular magnetic field such that only the ions with a desiredmass-to-charge ratio are able to travel through the analyzer for maximumtransmission through the mass resolving slit 117. Ions of the desiredspecies pass from mass slit 117 through deceleration stage 118 tocorrector magnet 119. Corrector magnet 119 is energized to deflect ionbeamlets in accordance with the strength and direction of the appliedmagnetic field to provide a ribbon beam targeted toward a work piece orsubstrate (100 in FIG. 3B) positioned on support (e.g. platen) 102. Insome embodiments, a second deceleration stage 122 may be disposedbetween corrector magnet 119 and support 102. The ions lose energy whenthey collide with electrons and nuclei in the substrate and come to restat a desired depth within the substrate based on the accelerationenergy. A mask 104 (shown in FIG. 3 b) is disposed proximate thesubstrate (shown in FIG. 3 b at 100) in a process chamber which housesplaten 102. The mask 104 may be referred to herein as a shadow orproximity mask. The mask has a plurality of apertures (105 in FIG. 3B)which allows portions of the ion beam aligned with the apertures totravel towards the substrate and blocks portions of the ion beam notaligned with the apertures 105.

The ion beam 101 has a height (Y direction) that is smaller than itswidth (X direction) as it travels toward the substrate (in the Zdirection). Since the height of the ion beam is smaller than the width,only a portion of the substrate is exposed to the ion beam. Thus, inorder to scan the entire substrate, the ion beam 101 (and consequentlythe mask 104) must move relative to the substrate or the substrate mustbe moved relative to the ion beam 101. However, if the ion beam and maskmove in order to scan the surface of the substrate, complicated coolingand grounding connections must accommodate this scanning movement in theprocess chamber where space is already at a premium. Thus, it is lesscomplicated to move the substrate with respect to the ion beam 101. Ascanning assembly 102 a is coupled to platen 102 and is configured tomove the substrate with respect to the ion beam. In particular, thescanning assembly moves the substrate at a first scan rate when firstportions of said substrate are aligned with the plurality of aperturesand at a second scan rate when second portions of said substrate arealigned with said plurality of apertures.

The scanning assembly is used to move the substrate at a variable speedin the Y direction as the ion beam implants a desired ion dose amount onthe substrate. Alternatively, the scanning assembly can move thesubstrate at a variable speed in the X direction as the ion beamimplants a desired ion dose amount on the substrate. In this manner, thescanning speed may slow down to have longer dwell times at particularportions of the substrate to heavily dope contact regions of a solarcell and move faster to have shorter dwell times to lightly dope exposedemitter regions of the cell.

FIG. 3B is a cross-sectional view of implantation through a mask. When aspecific pattern of ion implantation in a substrate 100 is desired, amask 104 may be placed in front of a substrate 100 in the path of an ionbeam 101. This mask 104 may be a shadow or proximity mask. The substrate100 may be, for example, a solar cell placed on platen 102 Typicalsubstrates used for solar cells are very thin, often on the order of 300microns thick or less. The substrate 100 is retained in position on theplaten 102 using electrostatic or physical forces. Although notrequired, it is preferable that the width of the substrate in the Xdirection is less than the width of the ion beam 101. However, no suchlimitation is preferred with respect to the orthogonal direction of thesubstrate.

The mask 104 has one or more apertures 105 that correspond to thedesired pattern of ion implantation. Use of the shadow mask 104eliminates process steps, such as screen printing or lithography,required for other ion implantation techniques. As previously stated,there is an advantage to having high dopant levels under the contactportions of the solar cell, such as seen in the regions 201 in FIG. 1.While a selective emitter solar cell is discussed, embodiments of thismethod may be applied to other solar cell designs. Point contacts willminimize the metal-to-silicon contact area. For these point contacts,dopant should be localized under the contacts 202 to provide anelectrical field to shield the contacts from minority carriers. Thedopant must be localized because highly-doped regions are detrimentalunder the passivated surfaces that may exist between the contacts 202.

Point contacts may be doped by implanting through a mask that has pointson it. However, the series resistance of a solar cell with such a dopingstructure may still be limited by the amount of dopant between thecontacts 202. In an implant through the mask 104, the mask 104 musttravel with the substrate 100 when the substrate 100 is scanned. Asmentioned above, this movement increases the difficulty in cooling andgrounding the mask 104. Additionally, a mask 104 with small apertures105 will block much of the ion beam 103 which decreases productivity anddevice throughput.

A fixed mask 104 having a plurality of apertures 105 is configured tocover the height (Y direction) of the ion beam 101. This may be, forexample, the height of a ribbon ion beam 101 or the maximum and minimumvertical extent of a scanned ion beam 101. Such a design providespositional definition in one dimension, such as the dimension in whichthe ion beam 101 is scanned (e.g. Y direction) or the long dimension ofa ribbon ion beam 101 (e.g. X direction). Positional definition in asecond dimension, such as the direction in which the substrate 100 isscanned, is provided by changing the scan speed of the substrate 100.

FIG. 4 is a front perspective view of a shadow mask 404 having sevenapertures 405 and is fixed (i.e., does not translate or scan) withrespect to the ion beam 101. While seven apertures 405 are illustrated,the mask 404 is not solely limited to seven apertures 405 and othernumbers are possible. An ion beam, such as ion beam 101, is incident onthe mask 404 throughout the implant process and the mask 404 isstationary or fixed relative to this ion beam. The substrate 100 isscanned behind the mask 404. Portions of the mask 405 a defined betweenthe apertures 404 and at the ends of the mask 405 b and 405 c block ionbeam from implanting on the substrate 100.

FIG. 5 is example of a corresponding scanning velocity profile for anexemplary substrate. This velocity profile has the substrate 100 movingfor a large percentage of the velocity profile at a first speed, but thesubstrate 100 slows down to a second speed when the ion beam 101 is oneach of the five evenly-spaced locations on the substrate 100. Inparticular, the substrate 100 moves at a first speed S₁ for an exemplarytime interval defined between t_(N) and t_(N-1) represented by reference501. The movement speed of substrate 100 changes to speed S₂ for anexemplary time interval defined between t_(N-1) and t_(N-2) representedby reference 500. Since the scan speed at S₁ is slower than the scanspeed S₂, the dose rate of ions from ion beam 101 on the surface ofsubstrate 100 at speed S₂ will be lower since the substrate is movingfaster through the ion beam. Thus, the dose implanted between these fivelocations on substrate 100 (represented by 501) will be lower than thedose at the five locations on the substrate (represented by 500) becauseof the higher scan speed S₁. This may be used to align the higher doserate locations of the substrate to the point metal contacts on anexemplary solar cell.

FIGS. 6A-6E illustrate results of the implant method corresponding tothe velocity profile of FIG. 5. In FIG. 6A the ion beam 101 implantssubstrate 100 through the apertures 405 in the mask 404 in a firstposition. Lower dose regions 601 correspond to that portion of thevelocity profile of FIG. 5 where the scan speed is faster (S₁). In FIG.6B, the ion beam 101 implants through the apertures 405 in the mask 404in a second position where the higher-dose implanted regions 600correspond to that portion of the velocity profile having a slower scanspeed (S₂). Thus, FIG. 6B illustrates the higher dose implanted regions600, lower-dose implanted regions 601 from the first position and theundoped regions 602 corresponding to the areas of the mask 405 a betweenthe apertures 405.

The substrate 100 may be scanned behind the mask 404 continuously.However, as seen in FIG. 5, the substrate 100 scans more slowly at thesecond speed when the ion beam 101 is implanting the higher-doseimplanted regions 600 as compared to the lower-dose regions 601 of thesubstrate 100. Thus, the substrate 100 moves faster at the first speedbetween the first position and second position (not illustrated) thanwhile implanting in the first position and second position illustratedin FIGS. 6A-6B. Since the mask is fixed with respect to the ion beam101, there is no need to move the mask with the substrate duringscanning of the substrate which provides increased manufacturingthroughput especially at lower dose levels.

FIGS. 6C-6E represent the results of ion beam 101 implanted throughapertures 405 in the mask 404 in a third position, fourth, position, andfifth position respectively corresponding to portions of the substrate100 as it moves in the exemplary Y direction. FIG. 7 is a frontperspective view of a substrate that results from the implant methodillustrated in FIGS. 6A-6E. As illustrated in FIG. 7, a series ofhigher-dose implanted regions 600 have been formed in the substrate 100with lower-dose implanted regions 601 in between. Undoped regions 602correspond to portions of the substrate that are aligned behind the maskareas 405 a, 405 b and 405 c between the apertures 405 and at theperiphery of the mask 404. The higher-dose implanted regions 600 maycorrespond to future point contacts in a solar cell that are addedduring a metallization step. Since the mask 404 is stationary or fixedin FIGS. 6A-6E, this results in less-complicated cooling and groundingconnections because the mask 404 does not move like the substrate 100.This also may improve the effectiveness and reliability of the coolingand grounding connections for the mask 404.

The substrate 100 of FIG. 7 could be produced by a shadow mask, such asmask 104, that scans with the substrate 100. However, the entiresubstrate 100 would have to remain in the ion beam 101 for the sameamount of time as each of the implant steps shown in FIGS. 6A-6E. Thismeans that the ion beam 101 dose required to perform the implants wouldbe increased by a factor of S_(h)/((B_(h))(n)) where S_(h) is the heightof the substrate 100, B_(h) is the height of the ion beam 101, and n isthe number of rows of the higher-dose implanted regions 600. For an ionbeam 101 with a height of 4 mm, a substrate 100 height of 156 mm, andfour rows of higher-dose implanted regions 600, the total dose could beten times lower using the method illustrated in FIGS. 6A-6E than animplant method with a scanning mask 104. This reduced dose results inincreased throughput and lower production costs. The reduced dose alsomay lead to less thermal load on the mask 404.

The lower-dose implanted regions 601 between the higher-dose implantedregions 600 may lower the series resistance of any resulting solar cell.Majority carriers must be transported from generation sites to thecontacts and the resistance these encounter during transportationreduces the output of a solar cell. For majority carriers that originatebetween the contact points, a slightly higher dose between the contactswill lower the resistance. This may result in better internal seriesresistance in the solar cell.

The present disclosure is not to be limited in scope by the specificembodiments described herein. Indeed, other various embodiments of andmodifications to the present disclosure, in addition to those describedherein, will be apparent to those of ordinary skill in the art from theforegoing description and accompanying drawings. Thus, such otherembodiments and modifications are intended to fall within the scope ofthe present disclosure. Furthermore, although the present disclosure hasbeen described herein in the context of a particular implementation in aparticular environment for a particular purpose, those of ordinary skillin the art will recognize that its usefulness is not limited thereto andthat the present disclosure may be beneficially implemented in anynumber of environments for any number of purposes. Accordingly, theclaims set forth below should be construed in view of the full breadthand spirit of the present disclosure as described herein.

1. A method of implanting ions into a solar cell substrate comprising:directing an ion beam through an aperture of a shadow mask toward asubstrate support configured to support a substrate; exposing a firstportion of said substrate, aligned with said aperture of said shadowmask, to said ion beam; moving said substrate support with respect tosaid ion beam at a first scan rate when said first portion of saidsubstrate is exposed to the ion beam; exposing a second portion of saidsubstrate, aligned with said aperture of said shadow mask, to said ionbeam; and moving said substrate support with respect to said ion beam ata second scan rate when said second portion of said substrate is exposedto said ion beam, wherein said first scan rate is slower than saidsecond scan rate.
 2. The method of claim 1 wherein said substrate is amaterial used to form a solar cell, said first scan rate correspondingto point contacts of said solar cell.
 3. The method of claim 1 furthercomprising fixedly positioning said shadow mask with respect to said ionbeam.
 4. The method of claim 3 wherein said shadow mask is fixed withrespect to a first dimension of said ion beam.
 5. The method of claim 4further comprising scanning said ion beam on said substratecorresponding to said first dimension.
 6. The method of claim 5 whereinsaid first dimension corresponds to a height of said ion beam.
 7. Themethod of claim 5 wherein said first dimension corresponds to a width ofsaid ion beam.
 8. The method of claim 1 further comprising aligning saidshadow mask orthogonally with respect to a path of said ion beam.
 9. Themethod of claim 1 providing said shadow mask with apertures that have aheight dimension greater than a height dimension of said ion beam.
 10. Amethod of implanting ions into a substrate comprising: directing an ionbeam through a plurality of apertures of a shadow mask toward asubstrate support configured to support a substrate; exposing firstportions of said substrate to said ion beam corresponding to saidplurality of apertures of said shadow mask; moving said substratesupport with respect to said ion beam at a first rate when said firstportions of said substrate are exposed to said ion beam; exposing secondportions of said substrate to said ion beam corresponding to saidplurality of apertures of said shadow mask; and moving said substratesupport with respect to said ion beam at a second rate when said secondportions of said substrate are exposed to said ion beam, wherein saidfirst scan rate is faster than said second scan rate.
 11. The method ofclaim 10 further comprising blocking said ion beam from reaching thirdportions of said substrate aligned with areas of said shadow maskbetween said plurality of apertures when said first and second portionsof said substrate are exposed to said ion beam.
 12. The method of claim10 further comprising aligning said shadow mask orthogonally withrespect to a path of said ion beam.
 13. The method of claim 10 furthercomprising fixedly positioning said shadow mask with respect to said ionbeam.
 14. The method of claim 10 wherein said second scan ratecorresponds to point contacts of a solar cell.